1. Field of the Invention
The present invention relates to an engine control unit to control the engine of a motorcycle having the drive wire system.
2. Description of Background Art
Several methods are known for controlling a throttle valve opening of an vehicle engine electrically by motors through the drive-by-wire system. One of such methods is disclosed in Japanese Patent Laid-Open No. Hei 5-79354. This disclosure is concerned with an electronic accelerator system or electronic throttle system which is provided with a mechanism wherein the throttle valve is mechanically moved a certain amount toward its open side as the accelerator pedal is pressed down. This mechanism is intended to prevent the throttle valve from being moved by the return spring to the closed position (which disables the operation of the engine) in case of a failure in the electronic system. Therefore, this mechanism permits a certain amount of intake air to flow into the engine, thereby to start the engine, even when an anomaly occurs in the electronic throttle system.
However, the mechanical structure disclosed in Japanese Patent Laid-Open No. Hei 5-79354, which is designed to keep the throttle valve slightly open in case of an anomaly, has the disadvantage of requiring a complex mechanism in and around the throttle body in spite of the electronic throttle system. Consequently, it is inevitably large in size, which prevents its adoption into a motorcycle which does not have sufficient space to accommodate additional parts.
Moreover, if the structure disclosed in Japanese Patent Laid-Open No. Hei 5-79354 is not adopted, the throttle valve is moved to the close position by the return spring in case of an anomaly. The result is that the engine which has stopped in the anomalous state does not suck in as much intake air as necessary for restarting but sucks in overrich intake air, because the throttle valve remains closed. This makes the engine hard to restart and prevents reverse driving. The engine incapable of restarting is particularly undesirable for comparatively large motorcycles equipped with a reverse system which permits the motorcycle to move backward while the engine is running.